The Organic Light Emitting Display (OLED) possesses many outstanding properties of self-illumination, low driving voltage, high luminescence efficiency, short response time, high clarity and contrast, near 180° view angle, wide range of working temperature, applicability of flexible display and large scale full color display. The OLED is considered as the most potential display device. The OLED can be categorized into two major types according to the driving methods, which are the Passive Matrix OLED (PMOLED) and the Active Matrix OLED (AMOLED), i.e. two types of the direct addressing and the Thin Film Transistor (TFT) matrix addressing. The AMOLED comprises pixels arranged in array and belongs to active display type, which has high lighting efficiency and is generally utilized for the large scale display devices of high resolution.
The AMOLED is a current driving element. When the electrical current flows through the organic light emitting diode, the organic light emitting diode emits light, and the brightness is determined according to the current flowing through the organic light emitting diode itself. Most of the present Integrated Circuits (IC) only transmit voltage signals. Therefore, the AMOLED pixel driving circuit needs to accomplish the task of converting the voltage signals into the current signals. In the general AMOLED pixel driving circuit, there are two thin film transistors and one capacitor, which is named the 2T1C pixel circuit. The first thin film transistor is named to be the switch thin film transistor, and employed to control the entrance of the data signal. The second thin film transistor is named to be the drive thin film transistor, and employed to control the current flowing through the organic light emitting diode. Thus, the importance of the threshold voltage Vth of the drive thin film transistor is quite obvious. Either of the positive or negative drift of the Vth will make different current flow through the organic light emitting diode with the same data signal.
All the thin film transistors manufactured according to prior art will have the phenomenon of the threshold voltage drift. Meanwhile, the threshold voltage drift also will happen after the organic light emitting diode is used in a long time, which leads to that the current passing through the organic light emitting diode is not consistent with the desired current, and the panel brightness cannot satisfy the requirement.
The drift of the threshold voltage in the general 2T1C circuit cannot be improved with adjustment. Thus, it is required to add a new thin film transistor or a new signal to weaken the influence due to the threshold voltage drift so that the AMOLED pixel driving circuit can have the compensation function. At present, most of the AMOLED pixel driving circuit, in which the traditional single gate thin film transistor is employed to be the drive thin film transistor adjusts the value of the data signal which is required to input by detecting the threshold voltage of the thin film transistor and then according to the drift level of the threshold voltage. However, after the single gate thin film transistor suffers the stresses of the voltage and the light irradiation, the threshold voltage commonly drifts forward and increases, and thus the data signal has to be increased accordingly to weaken the influence of the threshold voltage drift of the drive thin film transistor. The increase of the data signal enlarges the voltage stress of the drive thin film transistor in advance to speed up the threshold voltage drift of the drive thin film transistor and to form the vicious cycle.
Certainly, some AMOLED pixel driving circuits having the compensation function utilizes the dual gate thin film transistor to be the drive thin film transistor. The property of the dual gate thin film transistor is that the influence of the voltage and the light irradiation stress to the threshold voltage is smaller, and meanwhile, the threshold voltage and the top gate voltages appear to have the negative correlation trend. As shown in FIG. 1, which shows an AMOLED pixel driving circuit according to prior art, the AMOLED pixel driving circuit has the 4T2C structure, i.e. the structure of four thin film transistors plus two capacitors, comprising: a first thin film transistor T10, a second thin film transistor T20, a third thin film transistor T30, a fourth thin film transistor T40, a first capacitor C10, a second capacitor C20 and an organic light emitting diode D10. The first thin film transistor T10 employed to be the drive thin film transistor is a dual gate thin film transistor, and a top gate thereof is electrically coupled to a third node T′, and a bottom gate is electrically coupled to a first node B′, and a source is electrically coupled to an anode of the organic light emitting diode D10, and a drain is electrically coupled to a second node D′; a gate of the third thin film transistor T30 receives a second scan control signal S20, and a source receive a power source voltage VDD, and a drain is electrically coupled to the second node D; a gate of the fourth thin film transistor T40 receives a third scan control signal S30, and a source receives a data signal Data, and a drain is electrically coupled to the first node B′; one end of the first capacitor C10 is coupled to the first node B′, and the other end is grounded; one end of the second capacitor C20 is coupled to the third node T′, and the other end is grounded, and an anode of the organic light emitting diode D10 is coupled to the source of the first thin film transistor T10, and a cathode is grounded.
FIG. 2 is a sequence diagram corresponding to the circuit shown in FIG. 1. The working procedure of the AMOLED pixel driving circuit according to the sequence is divided into three stages: a pre-charge stage 10, a threshold voltage programming stage 20 and a drive stage 30. In the pre-charge stage 10, the first scan control signal S10 provides high voltage level, and the second scan control signal S20 provides high voltage level, and the third thin film transistor T30 is activated, and the power source voltage VDD charges the second capacitor C20 to VDD through the third thin film transistor T30 and the second third thin film transistor T20, and then the voltage of the top gate of the first third thin film transistor T10 is VDD to drop the threshold voltage of the first third thin film transistor T10, and the third scan control signal S30 provides low voltage level to deactivate the fourth thin film transistor T40, and the data signal Data cannot enter. In the threshold voltage programming stage 20, the first scan control signal S10 still provides high voltage level, and the second thin film transistor T20 is kept to be activated, and the second scan control signal S20 provides low voltage level, and the third thin film transistor T30 is deactivated, the third scan control signal S30 provides high voltage level, and the fourth thin film transistor T40 is activated, and the data signal Data provides a lower preset voltage Vpre entering the bottom gate of the first thin film transistor T10, and the first thin film transistor T10 is activated due to the low threshold voltage, and the voltage stored by the second capacitor C20 is released through first thin film transistor T10, the second thin film transistor T20 and the organic light emitting diode D10, and the threshold voltage of the first thin film transistor constantly increases until the first thin film transistor T10 is deactivated, and then, the threshold voltage is Vth=VBG−VS=Vpre−VOLED, wherein VBG represents the bottom gate of the first thin film transistor T10, and VS represents the source voltage of the first thin film transistor T10, and Vpre represents the data signal voltage at this stage, and VOLED represents the threshold voltage of the organic light emitting diode D10. In the drive stage 30, the first scan control signal S10 provides a low voltage level pulse signal, and the second thin film transistor T20 is deactivated, and both the second scan control signal S20 and the third scan control signal S30 provide high voltage level, and both the third thin film transistor T30 and the fourth thin film transistor T40 are activated, and the data signal Data is raised and enters the bottom gate of the first thin film transistor T10, and the first thin film transistor T10 is activated, and the organic light emitting diode D10 emits light.
After the threshold voltage programming stage 20, the threshold voltage of the first thin film transistor T10 is Vth=Vpre−VOLED, and according the formula of the current flowing through the thin film transistor:
                    I        =                ⁢                              β            ⁡                          (                                                V                  BG                                -                Vth                -                                  V                  S                                            )                                2                                        =                ⁢                              β            ⁡                          (                                                V                  Data                                -                                  V                  pre                                +                                  V                  OLED                                -                                  V                  OLED                                            )                                2                                        =                ⁢                              β            ⁡                          (                                                V                  Data                                -                                  V                  pre                                            )                                2                    wherein β is a constant coefficient related with the property of the thin film transistor.
The value I of the current flowing through the organic light emitting diode D10 is irrelevant with both the threshold voltage of the first thin film transistor T10 and the threshold voltage of the organic light emitting diode D10, and only is relevant with the voltage differences of the data signal Data in the drive stage 30 and the threshold voltage programming stage 20.
Such kind of AMOLED pixel driving circuit can compensate the threshold voltage of the drive thin film transistor and the threshold voltage of the organic light emitting diode but it requires adjusting the voltage value of the data signal of the input circuit for achievement, which makes the data signal more complicated and increase the stress influence to the drive thin film transistor.